One of the challenges for achieving optimum printability of paper has been the ability to control independently the surface and bulk properties of the paper. This is particularly true in the calendering process, in which the paper manufacturer attempts to improve the surface properties of the sheet but in doing so, affects the bulk properties. Calendering is known to densify the paper, which changes the physical properties (e.g., opacity, porosity and stiffness) of the paper.
In view of the foregoing, alternative methods for improving the smoothness of the board without sacrificing bulk and stiffness are of interest. Smoothness can be developed by allowing the cellulose fibers to replicate a smooth finishing surface. This can be accomplished by heating the fibers to a temperature higher than the glass transition temperature of the fibers and pressing the fibers to a smooth surface. On the other hand, bulk preservation is expected to be better at lower temperatures, where the web is relatively incompressible.
Almost 100% of the in-line printing of envelopes uses a water-based flexographic process and this constitutes more than 80% of all printing done on envelopes. The advantages to using the flexographic process are higher productivity rates and lower costs for the converter. The disadvantages include relatively poor print quality compared to off-line lithographic printing processes. Coated paper grades print well in a flexographic process, but due to ink drying issues and slipperiness, suffer productivity losses when compared to uncoated papers. Thus there is a demand among envelope converters for a sheet that runs like uncoated paper but prints like coated paper.
As used herein, "coated paper" refers to a paper product to which at least 8 grams per square meter of coating color solids have been applied to at least one surface at a coating station. Examples of common coating stations include blade coaters, rod coaters, short dwell applicator coaters, gate roll coaters, film press coaters, fountain coaters, and the like. The mixture of coating color will generally consist of (1) pigment(s) such as clay, calcium carbonate, titanium dioxide, and the like, (2) binder(s) such as modified starch, styrene butadiene rubber, polyvinyl acetate, vinyl acrylic, or polyvinyl alcohol, and (3) various functional additives such as dispersants, viscosity modifiers, crosslinking agents, lubricants, and the like. The resulting mixture is applied at a mixture solids content of 40% or greater by weight, and means are provided for controlling the amount of dry coat weight applied. A size press operation applying a mixture of starch and pigment to the sheet may or may not be used prior to the coating station.
The term "uncoated paper", as used in the written description and the claims herein, is defined as any paper product which has 0 to 8 grams per square meter of a starch or starch/pigment mixture solids applied to one or both sides of the web, but which does not undergo subsequent surface application as described above. "Uncoated paper" thus may or may not undergo treatment at the size press. If a starch or starch pigment mixture is applied at the size press, the solids content of the mixture for "uncoated" paper will be less than 40% by weight.
In order to attain the micro-smoothness required for good flexographic plate contact, conventional papermaking processes tend to compress the sheet, sacrificing bulk, stiffness and converting performance. Since envelope conversion generally requires only one side to be finished, a process which could treat one surface while preserving sheet bulk would be desirable.
Temperature gradient calendering is a known process where the surface of the paper is heated to a temperature higher than the glass transition temperature of the cellulose in the nip while the temperature of the sheet is substantially cooler. This process enables smoothness development with reduced bulk loss compared to regular machine calendering. In addition, surface moisturization can also be used to lower the glass transition temperature preferentially closer to the surface to develop smoothness with minimum bulk loss.
Soft calendering, another method of calendering used primarily for coated substrates, also relies on the temperature gradient calendering concept but the web that is being pressed against a hot surface in a nip is supported by a roll that has a deformable cover. This cover gives the paper a longer dwell time in the nip compared to hard steel nips and also allows the smoothness and gloss development to occur at relatively uniform density across the width of the paper. Soft calendering is an expensive option for existing machines and has limitations, such as cover delamination and cracking due to overheating.
Extended nip calendering extends the soft calendering concept to longer nip widths and reduces the operational problems. One type of extended nip calendering uses an endless band/belt over a backing roll to provide support for a paper web that is pressed against a heated cylinder. Another variation to this concept is to use a shoe instead of a roll as a backing for the paper web. The backing shoe provides longer nip widths and hence an increased dwell time.